The polyphenylene ethers and processes for their preparation are known in the art and are described in numerous publications including the Hay patents, U.S. Pat. Nos. 3,306,874 and 3,306,875. Also the Bennett and Cooper patents, U.S. Pat. Nos. 3,639,656, 3,642,699 and 3,661,848 describe processes for the preparation of polyphenylene ethers. All of these patents are incorporated by reference.
In general, the prior art processes involve the oxidative self-condensation of a monovalent phenolic precursor using oxygen and a catalyst comprising an amine-copper salt complex. Phenols which are polymerized by the process are monovalent phenols having substitution in at least the two ortho positions and hydrogen or halogen in the para position. By way of illustration, they correspond to Formula I: ##STR1## where X is a substituent selected from the group consisting of hydrogen, chlorine, bromine and iodine, Q is a monovalent substituent selected from the group consisting of hydrocarbon radicals, halohydrocarbon radicals having at least two carbon atoms between the halogen atom and the phenol nucleus, hydrocarbonoxy radicals and halohydrocarbonoxy radicals having at least two carbon atoms between the halogen atom and the phenol nucleus; and Q' and Q" are the same as Q and in addition, halogen, with the proviso that Q, Q' and Q" are all free of a tertiary alpha-carbon atom. Polymers formed from the above-noted phenols will therefore correspond to Formula II: ##STR2## WHERE THE OXYGEN ETHER ATOM OF ONE REPEATING UNIT IS CONNECTED TO THE PHENYLENE NUCLEUS OF THE NEXT REPEATING UNIT; Q, Q' and Q" are as above defined; and n is a whole integer equal to at least 50.
According to the process of the Hay patents, the formation of the polyphenylene ethers involves the self-condensation of the phenolic compound in the presence of oxygen and a catalyst system comprising an amine-basic salt complex. Various methods are reported for forming the complex catalysts starting with the copper salts. For example, a reducing agent can be used with a cupric salt to form the cuprous salt in situ, which in turn forms the amine-basic salt complex when admixed with the amine. Alternatively, the complex can be formed between an amine and a basic cupric salt formed by reacting cupric salts with an alkaline salt of a phenol, by treating a cupric salt with an ion exchange resin having exchangeable hydroxyl groups, by adding a base to a cupric salt, or by adding cupric hydroxide to a cupric salt. For full details regarding the preparating of such complexes from copper salts and amines and their use in the oxidative coupling of phenols, reference is made to the above-mentioned Hay patents.
In general, all of the prior art processes which have employed a copper catalyst have been carried out at atmospheric pressure. The Hay patents mention that subatmospheric pressure may be employed as a means of removing water that is formed in the reaction. There is no mention of the use of super-atmospheric pressure. U.S. Pat. No. 3,573,257 shows the use of a pressure of 2 kg/cm.sup.2 in connection with a manganese catalyst for the polymerization of a 2,6-xylenol. There is no mention of the use of a copper-amine catalyst in the disclosed process.
It has now been found that when pressures in the range of 25 psig-300 psig are employed in conjunction with a complex catalyst that is prepared from a cupric halide and a primary or secondary amine, the rate of oxidative polymerization will be substantially increased. This procedure has the obvious advantage that more efficient use of production facilities may be made. In addition, it has been found that the use of increased pressure increases the ratio of carbon-oxygen coupling as distinguished from carbon-carbon coupling which results in the undesirable by-product, tetramethyldiphenoquinone. The use of a pressure of 35 psig reduces the total amount of quinone by 25%.